1. Field of the Invention
This invention relates to the derivation of useful energy from impure steam containing hydrogen sulfide, and more particularly to the reduction of emission of sulfur-containing compounds to the environment as a result of or in connection with the derivation of useful energy from impure steam which contains both hydrogen sulfide and an alkaline acting component.
2. Description of the Prior Art
Useful energy such as heat, mechanical power and electricity can be derived from geothermal resources by obtaining geothermal steam and/or aqueous geothermal liquids from a subterranean geothermal reservoir and processing the geothermal fluid in surface facilities to extract thermal energy therefrom. Of particular interest is the generation of electricity by passing geothermal steam through a steam turbine/generator.
Most geothermal fluids, particularly those in the gaseous state, contain appreciable quantities of noncondensable gases comprising gaseous compounds such as hydrogen sulfide, carbon dioxide, ammonia. nitrogen, hydrogen, methane, boric acid, other substances comprising boron, arsenic or mercury, and the like. The presence of hydrogen sulfide in the geothermal fluid used for the derivation of useful energy often poses an environmental problem because only small amounts of hydrogen sulfide can be acceptably discharged into the atmosphere or hydrosphere.
Various processing procedures have been proposed for the abatement of hydrogen sulfide emissions resulting from or in connection with the derivation of useful energy from geothermal resources. One proposed method involves treating the motive geothermal steam upstream of the turbine to reduce the hydrogen sulfide content of the steam and then deriving useful energy from this pretreated geothermal steam. Such upstream pretreatment methods are disclosed in U.S. Pat. Nos. 4,123,506, 4,202,864 and 4,242,305 to Spevack and No. 4,163,044 to Woertz. Other proposed methods involve treating only the noncondensable gases separated from the steam turbine and condenser effluents to remove hydrogen sulfide from the downstream gases by means of conventional hydrogen sulfide removal processes, such as the Stretford process, before discharging the gases to the atmosphere. Such downstream gas treatment conventional processes are capable of converting large fractions of the hydrogen sulfide in the gases treated to elemental sulfur or other useful sulfur containing compounds. It has been found, however, that particularly where the impure steam contains an alkaline acting component, e.g. ammonia, a substantial fraction of the hydrogen sulfide in the effluents from the steam turbine and/or condenser will not readily separate into the gaseous-phase with the other noncondensable gases but will dissolve in the liquid-phase with the steam condensate and, as a result, this substantial fraction of the hydrogen sulfide bypasses the downstream gas treatment process and is eventually emitted to the environment from the liquid-phase steam condensate. Various methods have been proposed to chemically treat the effluents from the steam turbine and/or condenser to convert the hydrogen sulfide therein to chemically different sulfur compound such as sulfur, sulfate or other oxidation product, for example: by addition of a noncatalytic oxidant such as hydrogen peroxide; or by use of an aqueous-phase oxidation catalyst such as a ferric salt or a chelated ferric salt with air or oxygen; or to convert dissolved hydrogen sulfide to an insoluble metal sulfide by the process of U.S. Pat. No. 4,218,431 to Spevack.
A method directed to eliminate the need to chemically react the hydrogen sulfide dissolved in the steam condensate in a system deriving useful energy from geothermal steam has been proposed in U.S. Pat. No. 4,259,300 to Lieffers, which employs an upstream acid treatment of the impure steam for removal of alkaline acting components, e.g. ammonia, such as by the treatment disclosed in U.S. Pat. No. 4,123,506 to Spevack, in combination with conventional downstream processes for removing hydrogen sulfide from the noncondensable gases separated, from the steam turbine and condenser effluents. In the Lieffers method, because the alkaline acting components are removed from the impure steam upstream of the turbine, the hydrogen sulfide will pass through the turbine and condenser and tend to concentrate in the effluent gaseous-phase together with the other noncondensable gases and uncondensed water vapor. This effluent gaseous-phase, which is easily separated from the aqueous or liquid-phase steam condensate effluent after derivation of energy from the impure steam, is treated by a conventional process for removing hydrogen sulfide from gases such as noted above. The lieffers method, however, involves acid treatment of the entire supply of impure steam, which constitutes a large volume at high temperature and pressure, with consequent high operating cost and high equipment cost due to increased potential corrosion and other upstream operating requirements. Furthermore, the upstream removal of alkaline acting components from the impure steam without upstream removal of hydrogen sulfide, as disclosed in said U.S. Pat. No. 4,123,506, renders the impure steam flow substantially more corrosive to the downstream turbine and condenser equipment, thereby also increasing plant maintanance costs and reducing energy productivity.
A primary object of the present invention is to provide improved process and apparatus for reducing the emission of sulfur containing compounds to the environment from the utilization of hydrogen sulfide containing impure steam comprising alkaline acting components, e.g. ammonia, in connection with the derivation of useful energy therefrom.
Another object of the present invention is to provide simple but effective process and apparatus by which the emission of sulfidecontaining compounds to the environment from facilities used in connection with the generation of electricity from geothermal steam containing ammonia is abated by a novel combination employing known process steps or means for removing hydrogen sulfide from hydrogen sulfide containing gases.
Still another object of the present invention is to provide integrated energy derivation and pollution abatement process and apparatus by which useful energy can be efficiently derived from hydrogen sulfide containing impure steam comprising ammonia while substantially reducing the emission of sulfur containing compounds to the environment.
Additional objects of the present invention are to achieve one or more of the foregoing objects by employing novel combinations embodying chemical and physical processes downstream of the turbine and condenser, which avoid the disadvantages of the Leiffer upstream ammonia removal method. It is intended to achieve such objects by retaining alkaline acting components, e.g. ammonia, in the impure steam passing through the turbine and/or condenser, by eliminating the need for special large volume and high temperature and pressure equipment required for upstream impure steam treatment, and by effecting downstream treatment of the liquid-phase steam condensate to cause dissolved hydrogen sulfide to transfer from the liquid into the gaseous-phase effluent. The present invention thus permits substantially all of the hydrogen sulfide originally present in the impure steam to be reacted and removed by means of conventional processes for removing hydrogen sulfide from gases at relatively low temperature and pressure without any reduction in the available energy content of the impure steam, and thereby provide for the production of useful energy from impure steam with efficient pollution abatement at reasonable cost.
Further objects, advantages and features of the present invention will become apparant to those skilled in the art from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings.